Things that I looked at (or should have looked at) when deciding which (Cambridge) college to apply to:-
Usual offer given - based on 4A-levels or 3. Do you need As. will you get As? Personally I would rather get rejected in January than August so I went for a college that gave (relatively) low offers but it was hard to get an offer in the first place.
Proportion of state school in the college. If you are worried about balls etc then go for one with a high %age of state school.
Size of college and size of your subject - do you want to be one of 2 geographers or one of 8 or one of 40?
No of applicants per place?
Do you want to go to a women's only college?
Distance of college from town / where your subject is based? I used to be able to get out of bed at 8:55 and still make 9am lectures.
How sporty / musical your college is and you are. Even if you are sporty you may not want to go to a sporty college though - I was on the first team for about 8 sports at my college just for turning up and owning a pair of trainers....
How old (and therefore rich) the college is - more hardship funds at the rich ones.
Having said that on many levels it doesn't really matter. You will love your college as it will be your home. One of my friends had all sorts of sensible logical reasons for applying to our college. It was only when we were in second year and she was showing us them in the prospectus that she realised that she had Mis-read the chart and applied to the wrong one!